A Canadian firm (Cassidian) that is a major supplier of laser based defense systems (AN/AAR-60 MILDS) against heat-seeking missiles for helicopters and fixed wing aircraft has developed a similar system to defend ships, vehicles, and aircraft against laser guided missiles. The LDS (Laser Defense System) uses a computerized detection system to sense the target being hit (“painted”) by a targeting laser, calculates the location of the targeting laser, and then fires another laser at the source to spoil the aim of the human (or software) keeping the laser on the target. Without that precise targeting, the laser guided missile is now misguided and will usually miss.
The older AN/AAR-60 MILDS (Missile Launch Detection System) provides a similar protection against shoulder fired surface-to-air missiles (like the SAM-7 or Stinger). Once MILDS detects a threat, it then triggers a separate system that uses laser beams to confuse the heat sensor in the front portion of the missile. The heat sensor is chasing after the heat put out by the target (the engine as well as the metal skin of the aircraft).
A typical missile defense system has two components. First, there are 4-6 ultraviolet detection sensors (weighing 3-4 kg/6-9 pounds each) mounted on different parts of the helicopter to detect an approaching missile. These sensors are linked to a 3-5 kg (7-11 pound) computer that contains software for determining that the object is indeed a missile and where it is headed. The detection computer is hooked to a countermeasures system using a laser to confuse the missiles guidance system (that is mainly homing in the heat of the aircraft engines). The countermeasures component weighs 14-17 kg (30-50 pounds), depending on type or model.
For over a decade there was a debate over whether to equip helicopters with flares and chaff or systems that use a laser to confuse the missiles guidance system. The laser systems originally cost about 40 percent more than the flare based ones (which costs about two million dollars per aircraft). But the cost difference has been shrinking. The flare systems use a proven technology, while the laser based ones have slowly been gaining experience under combat conditions. Thus there's increasing enthusiasm for the laser. This is because it's harder for incoming missiles to get past lasers and because as long as you have electricity, your laser system has ammo. Flare systems can run out of flares.